In the US, Uconnect enables a number of "remote control" benefits for car owners, from remotely starting the engine and cooling the car down before you get in, to sounding the horn or flashing its lights to locate it in a busy car park. It also has functionality that can turn the car into a wi-fi hotspot.

To demonstrate software they created to exploit the flaw, Miller and Valasek had Wired writer Andy Greenberg drive a Jeep while they remotely commanded the car to do certain things.

Some of those things were fairly innocuous - blasting the air-conditioner or the stereo and cleaning the windscreen. However, other successful commands - killing transmission and the brakes - will have carmakers worried, particularly as more cars become smarter and internet-connected.

The researchers plan to withhold details of the vulnerability they exploited until Black Hat 2015 in December. "From an attacker's perspective, it's a super nice vulnerability," Miller told Wired.

They plan to leave some important bits from their presentation.

First, although the initial point of entry is via Uconnect, the attacker is limited at that point to playing with the dashboard functions and GPS.

The more serious remote takeover functions were performed by accessing a chip in the head unit (the screen) and overwriting its firmware. Wired reported this process will be kept secret.

The researchers have also shared the findings with Fiat Chrysler over the past nine months, leading to the release of the "software security update" by the carmaker.

It is not the first time Miller and Valasek have hacked connected-car systems to take over critical functions.